The invention relates to a servocylinder unit for adjusting an amount of fuel fed to a vehicle drive engine. The periodical "Ingenieurs de l'Automobile" [Automotive Engineers], Oct. 1984, page 115, discloses a servocylinder unit installed in a rod linkage located between a pedal and an adjusting lever of a diesel injection pump. A medium, e.g., compressed air acts on the piston of the servocylinder unit automatically against the bias of a pre-stressed compression spring when an excessive drive slippage occurs at the drive gears of a motor vehicle. The compression spring is partially compressed by a load acting on the piston, so that the length of the rod linkage changes, resulting in a reduction of the amount of injected fuel which the diesel injection pump feeds to a drive motor of the motor vehicle. If the load on the piston is increased to the extent that the compression spring is almost completely compressed, the piston provides for switching off of the diesel injection pump and stopping feed of the fuel to the drive motor. It has been recognized that a hard compression spring is favorable for a sensitive regulation of the drive slippage. However, this has the disadvantage that it makes the switching off of the diesel injection pump unnecessarily difficult because either a high load pressure is needed for the piston or a large piston diameter is needed which more constructional space and results in higher production costs.
DE-PS 33 23 563 disclose a control device for limiting the number of revolutions and speed of a drive motor of a motor vehicle. This control device likewise employs a servocylinder unit, which includes a piston and a pretensioned compression spring, and is installed a rod linkage of a diesel injection pump to reduce the amount of the injected fuel. This servocylinder is also used for switching off the diesel injection pump. A servocylinder and a valve arrangement located upstream thereof and forming a part of a control device, can likewise be used for reducing fuel delivery and accordingly the drive slippage.
German Pat. No. 3,122,666 discloses a servocylinder unit which is installed in a rod linkage and comprises an outer cylinder, an annular piston, an inner cylinder located in said annular piston, and a second piston displaceable in the inner cylinder. The servocylinder unit comprises two pre-stressed compression springs for displacing the annular piston relative to the outer cylinder and the second piston relative to the inner cylinder, respectively, to their respective initial positions. The outer cylinder has two control terminals for two opposite sealed load chambers cooperating with the annular piston and the second piston, respectively. The chamber cooperating with the annular piston provides for shorting of the rod linkage and, thus, for switching off of the injection pump. A loading of the piston located in the inner cylinder through a pedal controlled valve and a throttle associated therewith causes a gradual compression of the compressing spring to provide for a gradual increase in the amount of injected fuel to effect a noiseless acceleration of the vehicle. Thus, DE-A-3,122,666 teaches combining two independent arrangements each comprising a cylinder, a piston, and a pressure spring in a single unit.